The need to remove nitrogen oxides from such gases exists, for example, in the case of an ethylene unit designed to process olefins from an FCC (Fluidized Catalytic Cracking) unit. In the FCC unit, the carbon to carbon bonds of long-chain hydrocarbons are cleared, so as to produce the desired light olefins. When regeneration of the FCC catalyst is conducted with air, there are produced nitrogen oxides, such as NO and NO.sub.2. In addition, free oxygen is present. Especially at low temperatures of below 0.degree. C., such as occur in, for example, the low-temperature portion of an ethylene unit, NO reacts with the oxygen that is always present at least in traces so as to form N.sub.2 O.sub.3 having a boiling point of-102.degree. C. and into N.sub.2 O.sub.4 having a boiling point at -11.degree. C. These nitrogen oxides can react with olefinic hydrogens having conjugated double bonds, for example with butadiene, with the formation of explosive resins which accumulate in the low-temperature separation of the olefin unit. These problems are described in detail in the publication: H. Bauer, Noncryogenic and Safe Demethanization of FCC Off Gases, Linde Reports on Science and Technology, 55, (1995), pages 15-18.
In addition, the formation of NO in an FCC unit cannot be avoided at an economically reasonable cost. Previously known are non-cryogenic processes for extracting olefinic hydrocarbons by absorption in scrubbing agents; such processes, however, require expensive analytical and operational procedures to avoid the build-up of explosive resins, whereby an additional necessary investment in apparatus increases the cost of the process. Conversely, heretofore a process for selectively separating nitrogen oxides from FCC residual gases was unknown.